Methane is the major constituent of natural gas and also of biogas. World reserves of natural gas are constantly being upgraded and more and more natural gas is currently being discovered than oil. Because of the problems associated with transportation of a very large volume of gas, most of the natural gas produced along with oil, particularly, at remote places, and the methane produced in petroleum refining and petrochemical process are flared and hence wasted. Both methane and CO2 are green house gases, responsible for global warming. Hence, in future the flaring of natural gas produced during the oil production and methane produced in the petroleum refining and petrochemical processes would not be allowed and hence the methane or natural gas is to be converted to useful value-added and easily transportable product like liquid hydrocarbons of gasoline range. The conversion of methane directly to higher hydrocarbons and aromatics is extremely difficult. If technologies are made available for the conversion of the natural gas or methane to easily transportable less volatile value added products such as aromatic hydrocarbons, a far reaching economic impact can be achieved which will also lead to exploration of more gas-rich field and also natural gas hydrates increasing the natural gas reserves.
Gasoline and aromatic hydrocarbons are an important commodity in the petroleum and petrochemical industries. The most commercially important aromatics are benzene, toluene, ethyl benzene and xylenes. Aromatics are currently produced by catalytic reforming of various petroleum feed stocks and catalytic cracking of naphtha. Aromatics can also be produced by catalytic conversion of alcohols (particularly methanol), olefins or lower alkanes (particularly propane, butanes or LPG). The catalyst used in these processes (methanol-to-gasoline Mobil's MTG process, olefins-to-gasoline-and-distillate or MOGD or M2 forming, both developed by Mobil Oil, and LPG-to-aromatics conversion process or Cyclar Process developed by UOP) belong to the pentasil zeolite family, particularly that having ZSM-5 structure.
An oxidative activation of methane for converting it directly to C2-hydrocarbons, ethane and ethylene, is known in the prior art and it is described in a book {E. E. Wolf “Methane Conversion by Oxidative Process: Fundamental and Engineering Aspects” Van Nostrand Trinhold Catalysis Series, New York, (1992)} and also in a number or review articles {J. R. Aderson, Appl. Catal., 47 (1989) 177, J. S. Lee et. al., Catal. Rev. -Sci. Eng., 30 (1988) 249; G. J. Hutchings et. al., Chem. Soc. Rev., 18 (1989) 25; J. H. Lunsford, Catal. Today 6 (1990) 235; J. H. Lunsford, Angew. Chem. Intl. Ed. Engl. 34 (1995)970}.
According to a recent U.S. Pat. No. 5,336,825 (1984) of Choudhary V. R. and co-workers, methane can be converted to gasoline range hydrocarbons comprising aromatic hydrocarbons by carrying out the conversion of methane in the following two steps. Step (i): Catalytic oxidative conversion of methane to ethylene and minor amounts of C3 and C4 olefins in presence of free oxygen using a basic solid catalyst at a temperature preferably between 600° C. and 850° C. Step (ii): catalytic conversion of ethylene and higher olefins formed in the step (i) to liquid hydrocarbons of gasoline range over an acidic solid catalyst containing high silica pentasil zeolite, using product stream of the step (I) as the feed.
In the other multistep processes as described in Eur, Pat. Appln. EP 516,507 (1992) and Fr. Appl. 91/6,195 (1991), a methane rich fraction of natural gas is selectively oxidized, mixed with a C2+ hydrocarbon rich natural gas fraction, pyrolized and then the mixture is aromatized with a catalyst based on zeolite and gallium.
All the prior art teach the process for the oxidative activation of methane involves the following undesirable highly exothermic methane combustion reactions:CH4+2O2→CO2+2H2O  (1)CH4+1.5O2→CO+2H2O  (2)
Hence, these processes are hazardous in nature. Moreover, in these processes, undesirable carbon oxides, CO and CO2, are formed thus reducing the product selectivity and also creating environmental pollution problems and global warming by the release of CO2 in the atmosphere.
High temperature non-oxidative conversion of methane to C2+ hydrocarbons is known in the prior art.
It has been known for a long time that methane and natural gas can be pyrolytically converted to benzene at temperatures above 899° C. (1659° F.), preferably above 1200° C.
A paper on “High Temperature Synthesis of Aromatics Hydrocarbons from Methane” published in Science 153 (1966) 1393 disclosed that aromatic hydrocarbons can be prepared from methane by contact with silica at 1000° C. The yield of hydrocarbons was in the range of 4.8–7.2% based on the methane used in the single pass at a gas space velocity of 1224 h−1.
More recent, a non-oxidative activation of methane by dehydrogenative coupling of methane over active carbon at temperature ≦1100° C. has been reported by H. Yagita et. al. [H. Yagita et. al., in Environmental Catalysis, G. Centi et. al. Eds. SCI Publication, Rome, 1995, page 639–642].
U.S. Pat. No. 4,814,533 (1989) discloses a continuous catalytic process for the production of higher molecular weight hydrocarbons rich in ethylene or aromatics or both from lower molecular weight hydrocarbons or methane in which a lower molecular weight hydrocarbon containing gas is contacted in a reaction zone with a higher molecular weight hydrocarbon synthesis catalyst at a temperature greater than 1000° C.
A recent Japanese Patent, Jpn. Kokai Tokkyo Koho JP. 07,155,600 (1995), discloses a process for the preparation of reaction media for aromatization and preparation of aromatics from methane at high temperature. The reaction media, which is prepared by thermal decomposition of cyclohexane at 1050° C., was fed with methane at 1050° C. for 2 h to give benzene in 54.7% selectivity at 30.9% conversion.
Because of the requirement of a high temperature for the conversion of methane and also due to the extensive coke formation at the high reaction temperature, the above processes based on the non-oxidative conversion of methane are difficult to practice and hence uneconomical.
Catalytic aromatization of methane in the absence of O2 using zeolite catalyst is also known in the prior art.
U.S. Pat. No. 4,727,206, GB Patent 8531687 and European Patent Application No. 0 228 267 A1 disclosed the aromatization of methane by contacting with gallium loaded zeolite containing group VII B metal or metal compound as a catalyst at a temperature from 600° C. to 800° C., preferably from 650° C. to 775° C., in the absence of oxygen. However, the conversion of methane into aromatics and the yield of the aromatics reported in the examples of these patents, are very low. At a weight hourly space velocity of 1.0 and absolute pressure of 7.0 bar, methane conversion at 675° C., 700° C. and 750° C. was 3.6 wt. %, 4.9 wt. % and 8.3 wt. %, respectively and aromatics yield was 2.0 wt. %, 2.53 wt. % and 2.95 wt. %, respectively. Because of the very low aromatics yield even at a temperature as high as 750° C., this process cannot be economically practiced.
A U.S. Pat. No. 5,026,937 (1991) discloses a process for the aromatization of methane using a catalyst comprising about 0.1 to about 2 wt. % gallium containing ZSM zeolite and phosphorus-containing alumina, at a gas hourly space velocity of 400–1500 h−1 at relatively low temperature conditions. As per the illustrated example of this process, when the catalyst was contacted with a stream of 100 mole % methane at a flow rate of 1.4 h−1 LHSV (liquid hourly space velocity) at 750° C. and at atmospheric pressure, the overall methane conversion was 3.5 mole % in the single pass, the selectivity to C2+ hydrocarbons was 72%, and the selectivity to coke was 28%. Because of the very low methane conversion even at 750° C. and low space velocity and also due to the very high selectivity to coke, this process is not economical. Because of the extensive coke formation, this process is also difficult to practice on a commercial basis.
Although aromatization of methane at a temperature below 700° C. is desirable for making the conversion of methane-to-aromatics process commercially more feasible, the aromatization of methane alone at the low temperatures is not at all thermodynamically possible. At a temperature below 700° C., the conversion of methane to benzene proceeds according to following reaction,6CH4→C6H6+9H2  (3)and involves a very large free energy change, ΔGr. The value of ΔGr, which is greater than 35 kcal per mole of benzene formed at or below 700° C., is much larger than zero. This high thermodynamic barrier does not allow the formation of benzene from methane at the lower temperatures. Hence, for the conversion of methane to aromatics at or below 700° C., it is necessary to find ways for overcoming the thermodynamic barrier and especially for the non-oxidative conversion of methane, which is the most inert of all of the hydrocarbons, at lower temperatures.
A number of U.S. Pat. Nos. 3,928,483 (1975), 3,931,349 (1976), and 4,05,576 (1977), 4,046,825 (1977), and 4,138,440(1979), assigned to Mobil oil corporation disclosed process for the production of gasoline from methanol, other alcohol and ether, using shape selective ZSM-5 zeolite catalyst. A commercial plant based on Mobil's methanol-to-gasoline (MTG) process, involving production of methanol from methane via syngas route: methane steam reforming to syngas and syngas conversion to methanol, was also successfully operated in New Zealand in 1985. However, the process economics was then not favourable to sustain the gasoline production. Since the commercial plant could not be operated economically, it was shut-down [Ref. J. Haggin, Methane-to-Gasoline Plant Adds to New Zealand Liquid Fuel Resources, Chemical & Engineering News page 22, Jun. 22, 1987; J. H. Lunsford, The Catalytic Conversion of Methane to Higher Hydrocarbons. Catal. Today, vol. 6 page 235, (1990)]. It is therefore of the great challenge to inventors to develop not only technically feasible but also economically feasible process for the conversion of methane to higher hydrocarbons by finding a novel way for non-oxidative activating chemically inert methane, particularly at lower temperatures, below 700° C.